Two principal types of harmonicas are the simple harmonica (which typically consists of eight or ten holes or cavities each of which can produce two notes, one draw-note and one blow-note), and the slide chromatic harmonica (which consists, in effect, of two separate simple harmonicas, one above the other). In slide chromatic harmonicas, one instrument is tuned a half-step higher than the other and the user switches from one to the other by depressing or releasing a movable slide.
Both types of harmonicas are available in twelve different keys, but for each key the progression of notes is generally the same. The standard arrangement of notes for a ten cavity simple harmonica is shown in FIGS. 1 and 2; that of two "simple harmonicas" in a typical slide chromatic harmonica is somewhat different.
In each cavity of the simple harmonica of FIGS. 1 and 2, the pitch of the draw- or blow-reeds is higher than that of the corresponding reed in the cavity to the left, and lower than that of the corresponding reed in the cavity to the right. Exhaling or blowing across any three adjacent holes of any of three sets of cavities (i.e., the first through third, fourth through sixth and seventh through ninth cavities) will produce a major triad (1-3-5) of the blow key-note (typically the note of the blow-reed of the first cavity, F in FIG. 1). The chord of the fourth through sixth cavities is one octave higher, and that of the seventh through ninth cavities is two octaves higher, than that of the first through third cavities. The seventh chord (1-3-5-.sup.b 7) of the draw key-note (typically the note of the draw-reed of the second cavity, C in the harmonica of FIG. 1), is produced by inhaling or drawing across the key-note cavity and the next three adjacent cavities (i.e., by drawing on the second through fifth cavities).
The simple harmonica of FIGS. 1 and 2 was originally designed to play European folk songs in the "blow mode," and an arrangement which produced the major triad chord of the blow key-note (the "blow" chord) and a dominant seventh chord of the blow- key-note (the "draw" chord) was satisfactory. These were the only two chords produced, however, and this arrangement has limited the types of music that may be played on the instrument.
Around the 1920's, the playing perspective and orientation began to focus on the "draw mode," in which songs were played in the key of the draw chord instead of that of the blow chord. There were a number of advantages to the "draw mode" approach.
One principal advantage was that blues and boogie woogie, popular at the time, played easily in the "draw mode"; they are based on the mixolydian scale which was most easily played in the draw key and features the flat seventh (.sup.b 7) of the scale, the primary "blue note." A second advantage is that, because of the "bending principle" discussed below, the draw reeds in the first through sixth cavities could be "bent down" in pitch; "bending" has become a hallmark of blues and modern playing. Because it is more expressive, the "draw mode" has prevailed in modern harmonica playing to the present day. However, the available harmonica instruments have placed considerable restrictions on the notes and chord progressions available.
U.S. Pat. No. 4,237,766 to Marshall discloses harmonicas having somewhat different arrangements which enable playing a few chords not generally available in traditional harmonicas, but each blow-note arrangement is such that any three adjacent holes produce the same major (1-3-5), minor (1-.sup.b 3-5), or diminished (1-.sup.b 3-.sup.b 5) triad of the blow key-note, and the only chord produced by the draw-notes is a major seventh, a minor seventh, or a diminished seventh flatted ninth chord of the draw key-note.